Jet engines create thrust by directing a high energy exhaust stream from an exhaust nozzle. Typically, a jet engine accepts air through an inlet and compresses the air in a compressor section. The compressed air is directed to a combustion chamber, mixed with fuel, and burned. Energy released from the burning fuel creates a high pressure in the combustion chamber which forces the air through a turbine section and into an exhaust chamber. The high pressure air is then forced from the exhaust chamber through a nozzle, where the air exits the engine. Typically, as the air passes through the throat of the nozzle, it expands and accelerates from subsonic to supersonic speeds, essentially translating the energy of the exhaust flow from a pressure into a velocity. The energy level of the air in the exhaust chamber generally relates to the velocity of the air as it exits the nozzle. The greater the velocity of a given mass flow of air exiting an engine, the greater the thrust created by the engine.
Military aircraft commonly augment the energy level of the air in the exhaust chamber by using an afterburner. Afterburners add fuel to the exhaust chamber and ignite the fuel in the exhaust chamber, which increases the temperature and pressure of the exhaust flow. Although the energy added by afterburned fuel can greatly increase the thrust of the engine, the increased energy level can also have several adverse effects on the engine. First, increased pressure in the exhaust chamber can slow the flow of air through the compressor section and turbine section, causing the engine to stall. Second, the increased temperature and pressure in the exhaust chamber can overheat the walls of the exhaust chamber leading to a failure, such as a burn through of a wall, and related dangers.
To alleviate these difficulties, jet engines with afterburners typically use variable geometry nozzles to throttle the exhaust flow from the exhaust chamber. When afterburner is initiated, the circumference of the nozzle's throat is increased to increase the cross-sectional flow area through the throat. This increased cross-sectional area allows air to more easily escape from the exhaust chamber, thus decreasing the pressure in the exhaust chamber and, as the air expands and leaves the exhaust chamber, also decreasing the temperature of the air. Modern afterburning jet engines with variable geometry nozzles can require as much as a two-fold increase in cross-sectional throat area to maintain constant engine flow and back pressure in response to the extra thermal energy added by afterburning.
Although variable geometry nozzles allow the use of afterburner, they also have many inherent disadvantages which penalize aircraft performance. For instance, a variable geometry nozzle can make up a significant portion of the weight of an engine. Such nozzles are typically made of large, heavy metal flaps which mechanically alter nozzle geometry by diverting exhaust flow with physical blockage, and thus have to endure the high temperatures and pressures associated with exhaust gases. In the iris-type nozzles typically used on afterburner-equipped engines, the actuators used to adjust the nozzle flaps to appropriate positions in the exhaust flow tend to be heavy, expensive and complex because of the forces presented by the exhaust flow which the nozzle flaps must overcome. Further, the nozzle flaps typically constrict the exhaust flow by closing and overlapping each other, which allows hot air to escape between the flaps. These leaks cause reductions in thrust. Variable geometry nozzles are also difficult to implement on exotic nozzle aperture shapes typical of advance tactical fighter aircraft.
Attempts to reduce the disadvantages of variable geometry nozzles have had limited success. The state-of-the-art tactical aircraft is the Lockheed F-22 Raptor. The Raptor employs a two-dimensional variable geometry nozzle that can vector or turn the exhaust flow of the Raptor's engine to provide directional thrust control. Although the two-dimensional nozzle flaps of the Raptor provide better infrared and radar cross section characteristics than can be obtained from typical iris-type nozzles, even the Raptor's advanced system suffers from the above-mentioned disadvantages. For instance, air can leak along the intersection of the two-dimensional nozzle flaps, introducing inefficiency.
Attempts to use a fixed geometry nozzle with afterburning engines have met with only limited success due to the difficulty of maintaining flow through the engine when an overpressure is created by afterburner initiation. For instance, U.S. Pat. No. 5,406,787 issued to Terrier uses an additional compression stage to vary pressure during engine operation and afterburning to counteract temperature variations created by the afterburner in the exhaust chamber. However, this system requires modification to the engine and other complexities such as a control program to monitor and adjust pressure produced by the compression section.
Another method for using a fixed geometry nozzle with a jet engine is to inject a secondary flow of high pressure air across the primary flow as the primary flow passes through the nozzle, as is explained in "Conceptual Development of Fixed-Geometry Nozzles Using Fluidic Injection for Throat Area Control" AIAA-95-2603 and "A Static Investigation of Fixed-Geometry Nozzles Using Fluidic Injection for Throat Area Control" by J. A. Catt and D. N. Miller, AIAA-95-2604, July 1995. The secondary flow can partially block the exhaust exiting through the nozzle to decrease the flow through the nozzle when needed to increase the pressure within the exhaust chamber. When an overpressure exists in the exhaust chamber, the cross flow can be reduced or eliminated to increase the flow through the nozzle.
Although the injection of a secondary cross flow will support a fixed geometry nozzle in an afterburning jet engine, this method also introduces inefficiencies to the engine's operation. For instance, the amount of afterburning may be limited due to the lower effectiveness of secondary injection compared to the effectiveness of variable geometry nozzles. Also, injection of air across the flow of the exhaust tends to use a large amount of high pressure air to obtain effective nozzle blockage. Thus, injection can introduce inefficiency because the total momentum of the exhaust flow is decreased by the decreased flow from the compressor section into the combustion section if compressed air is bled from the compressor section for injection. This inefficiency can result in a reduced range of operations for a given fuel supply, and reduced power for a given fuel flow.